1. Field of the Invention
The present invention relates to a flexible wiring board used in a liquid discharge head or the like mounted on a recording device such as an ink jet printer, and to the liquid discharge head.
2. Description of the Related Art
With respect to a method for installing a liquid discharge element substrate for a liquid discharge head mounted on an ink jet printer or the like, a connection method is known in which a flexible wiring board is superimposed on the liquid discharge element substrate to perform inner lead bonding (ILB), as disclosed in Japanese Patent Application Laid-Open No. 2001-130001.
FIG. 10 is a diagram illustrating the ILB according to a conventional example. An electric wire tape 120, which is the flexible wiring board, uses a base film 121 configured of an insulating material such as polyimide or the like as a substrate. On the base film 121, a sprocket hole (not shown) for locating a film transfer position and a device hole 122, which is an opening for inserting a recording element substrate 101 (101a, 101b) therein, are formed. To the surface of the base film 121, conductive metal foil such as copper foil or the like is attached. This metal foil is patterned into a desired shape using a photolithography technique, and wiring such as an inner lead 123 projected into the device hole 122 and an outer lead (not shown) are formed thereon. The surface of the metal foil after the patterning operation is subjected to plating treatment of, for example, gold, tin, solder or the like, and further, an area where a metal surface should not be exposed is covered with a protective layer of a resist, for example.
When the inner lead 123 of the electric wire tape 120 and an electrode pad 102 of the recording element substrate 101 are electrically connected, a bump 103, which is a projection of metal, is provided beforehand on the electrode pad 102 of the recording element substrate 101. Then, the inner lead 123 to be connected is positioned right above the bump 103, and the inner lead 123 is bonded to the bump 103 from the upper part of the inner lead 123 using a bonding tool. At this time, the recording element substrate 101 is fixed on a bonding stage by vacuum adsorption or on a secure support 110 by an adhesive or the like so as to obtain an excellent bonding state.
Normally, such ILB methods are, broadly speaking, divided into two methods. One is a gang bonding method in which an entire inner lead and bump are connected in a lump with the bonding tool for each recording element substrate. Another method is a single point bonding method in which the inner lead and the bump are connected separately, selectively, and successively, one after another. In either method, in bonding the inner lead and the bump, the bonding tool has to be heated to a relatively high temperature. Generally, in the single point bonding method, the heat is applied to a temperature of approximately 200° C. When the bonding is performed by the gang bonding method, the bonding tool is required to be heated to a high temperature of approximately 500° C.
As described above, the bonding between the bump 103 and the inner lead 123 is performed by the application of heat in a high temperature condition. Accordingly, the base film 121, mainly composed of an insulating organic resin such as polyimide or the like, and the inner lead 123, mainly composed of copper (Cu), are bonded to the bump 103 in a thermally expanded state. In particular, the thermal expansion of the base film 121 is larger than the inner lead 123, and thus, in the vicinity of a heating spot shown in FIG. 10, the base film 121 expands in a direction of widening the device hole 122 as indicated by an arrow A. After bonding, in a cooling process in which the electric wire tape 120 is separated from the bonding tool, discharged from a heating stage, and cooled to an ambient temperature, the electric wire tape 120 contracts so as to narrow the device hole 122 from the thermally expanded state to a normal state as indicated by an arrow B shown in FIG. 10. As a result, stress is applied to a bonding section between the inner lead 123 and the bump 103. The stress is affected by the volume of the base film 121. When the stress exceeds bonding strength between the electrode pad 102 and the bump 103 or between the bump 103 and the inner lead 123, a peeling phenomenon occurs at the bonding section.
In particular, in the case of an ink jet recording head (liquid discharge head) in which, for example, a color recording element substrate 101a and a black recording element substrate 101b are installed in a piece of the electric wire tape 120 as shown in FIG. 10, this phenomenon occurs for the reason described next.
An impact precision of a droplet, such as ink or the like, discharged from the recording element substrates 101a and 101b determines recording quality. Therefore, for example, two recording element substrates 101a and 101b for color and black are required to be fixed on the support 110 at a highly accurate position. Therefore, if the inner lead 123 of the electric wire tape 120 and the electrode pad 102 of each recording element substrate 101 are connected beforehand and then fixed on the support 110, each of two recording element substrates 101a and 101b is moved relative to an appropriate position after completion of the connection. In this step, the above-described ILB bonding section can be broken.
Accordingly, in a process for manufacturing the ink jet recording head, a method to be employed is to fix each recording element substrate 101 on the support 110 in an accurate position beforehand. Thereafter, the positioning is accurately performed so as to connect the inner lead 123 of the electric wire tape 120 and the electrode pad 102 of each recording element substrate 101, and in this state, the ILB is performed.
FIGS. 11A to 11D illustrate a state in which stress is applied to the ILB bonding section in such configuration. As shown in FIG. 11A, the electric wire tape 120 and the recording element substrate 101 are securely fixed on the support 110 in a highly accurate position to each other. During the ILB operation, heating is carried out in this state, and this causes a difference in thermal expansion between the recording element substrate 101, mainly composed of Si, and the base film 121 of the electric wire tape 120, mainly composed of polyimide. As a result, the inner lead 123 pasted on the base film 121 having a larger thermal expansion amount moves from the electrode pad 102, which was accurately positioned in an initial condition. Accordingly, as shown in FIG. 11B, a reciprocal deviation from the initial position occurs. Thus, practical bonding using the bonding tool is performed in a state deviating from the initial position as shown in FIG. 11C with a nick mark 122a. 
After completion of the bonding, the base film 121 and the inner lead 123 are discharged from the heating stage and are cooled to an ambient temperature. Then, the base film 121 and the inner lead 123 contract, and are urged to return to the initial stage as shown in FIG. 11D. The whole stress generated at this time is applied to the bonding section, which is subjected to the ILB, thus in the worst case, the bonding section is broken.
With respect to a tape carrier used for packaging a general semiconductor chip, for example, Japanese Patent Application Laid-Open No. 5-275498 discloses a method in which for the purpose of preventing the failure of the packaging caused by thermal expansion, a notch part is formed on each corner of the device hole to divide the surrounding of the inner lead.
Japanese Patent Application Laid-Open No. 5-275498 discloses the tape carrier which is scrapped after completing the connection of the inner lead. Therefore, even if the surrounding of the device hole is notched, no problem arises. However, in the case of the flexible wiring board, if the notch part or the like is formed on the surrounding of the device hole, the wiring cannot be carried out in this area. Accordingly, the size of the flexible wiring board increases. Further, when the flexible wiring board on which the notch part or the like is formed is installed on the recording element substrate of the ink jet recording head, there is concern that an electrical shorting between adjacent wires, or the corrosion of the wire or the like, can be caused by a liquid such as ink entering the notch part. As a measure against this, additional processes or treatment for protecting the exposed portion of a wire are further required.